This invention relates to feeding devices and, more particularly, to a pin wheel feeding device of the type generally used to positively feed marginally punched record material past a work station.
Pin wheel feeding devices of the above type have been known for quite some time (see, for example, U.S. Pat. Nos. 2,128,924 and 2,815,672). Due to the speed and versatility of modern day computers and computer output printers, contemporary pin wheel feeding devices should have the characteristics of precision and reversibility. Reversibility is highly desirable in printing machines when used to plot graphs, curves, charts and the like.
The pin wheel feeding devices disclosed in the aforementioned patents are not designed with a feed reversal capability. Feed reversal requires that pins project from the pin wheel housing at two locations, i.e. one adjacent the point of entry of the record material onto the housing, usually forming part of or connected to a rotatable platen, and one adjacent the point of exit of the record material from the housing-platen. The former set of pin projections feed in the forward direction and the latter in the reverse direction. It is clear that if only the former set of pin projections were present, as is the case in the above mentioned patents, the record material might lose tautness and be lifted off the platen during reverse feeding. This, of course, is entirely unacceptable.
The predominance of pin wheel feeding devices currently in use today, whether having a reversal capability or not, are generally characterized by a cylindrical housing capable of being rotated about its axis, the housing having a peripheral wall containing a plurality of openings formed therein, each opening lying along a radial line extending from said axis. A plurality of substantially identical pins are disposed within the housing in respective alignment with the plurality of openings, each pin capable of being moved along the respective radial line from a first position in which it is substantially entirely located within the housing to a second position in which it projects from the respective aligned opening. Each pin includes a pair of opposing guide members, the point on each guide member located closest the opposing guide member lying substantially along the respective radial line. A cam member is disposed within the housing and is capable of being fixed in position relative to the housing, the cam member having a track formed thereon with which each of the pins is engaged with the pair of guide members thereof being located on either side of the track. The pins follow the path of the track during rotation of the housing relative to the cam member. The track has a first segment capable of maintaining a pin engaged therewith at its first position, a second segment capable of maintaining a pin engaged therewith at its second position, and a third segment intermediate and contiguous with the first and second segments for enabling a pin engaged therewith to move between its first and second positions during rotation of the housing.
Reversible pin wheel feeding devices of the above type are further characterized in that the cam member has a track including a pair of first segments as above defined. The cam member is so mounted such that the pair of first segments are respectively adjacent the entry and exit locations of the record medium so that the pins will project from the housing at these locations.
A major problem with this type of reversible pin wheel feeding device stems from the fact that there is "lost motion" of the pins engaged with the first segments of the cam track. More specifically, the distance between the guide members on each pin engaged with a first segment is larger than the width of such first segment,, thereby leaving a space between the first segment and one of the pair of guide members, depending upon the direction of rotation of the housing. When this direction is reversed, the pin will not start following the track until such one guide member is moved into contact with the track, thereby resulting in a "lost motion". Lost motion is inconsistent with the precision requirements of most contemporary high speed printers and can lead to mishandling and damaging of the record medium.
The reason the cam track's first and second segments are narrower than the spacing between the guide members is due to the fact that the track is formed of a uniform width, which width is determined by the minimum width required for free movement of the pins along the third segment, above defined. More specifically, on such third segment, a pin is changing positions and thus is no longer moving along an arcuate path at a predetermined radius, which is normally the case with movement along the first and second segments. Nonetheless, the guide members on the pin continue to be aligned with the respective radial line from the axis of rotation of the housing, thereby decreasing the requisite width of the track at such third segment in order to allow free movement.
It would be desirable, therefore, to provide a pin wheel feeding device of the above general type where lost motion is substantially reduced during reversal of rotation, thereby making the device especially desirable for reversible feeding.